(1) Field of the Invention
The present invention relates to a cathode structure, an electron gun including the cathode structure, and a cathode ray tube including the electron gun.
(2) Description of the Related Art
A cathode structure is a component of an electron gun that is included in a cathode ray tube, and it is desired that the cathode structure is made as short as possible in a direction of a tube axis in order to reduce a size of the electron gun in the direction of the tube axis and the cathode ray tube that includes the electron gun. In terms of power consumption, it is also desirable that a heater for heating an electron-emitting material may be heated up as efficiently as possible.
An example of such cathode structures is disclosed in Japanese Laid-Open Patent Application No. 2001-202898, and a perspective view thereof is illustrated in FIG. 1A.
As shown in FIG. 1A, a cathode structure 202 comprises a cylindrical metal cup 204, a circular columnar pellet 206 incorporated in the metal cup 204, and a circular columnar heater 208. The metal cup 204 and the heater 208 are combined in a manner such that supporting metal wires 210 and 212 are positioned so as to cross each other between the metal cup 204 and the heater 208.
The pellet 206 is made of a porous refractory material impregnated with an electron-emitting material primarily composed of barium oxide (BaO). When the pellet 206 is heated by the heater 208, thermal electrons are emitted from an exposed surface of the pellet 206. The supporting metal wires 210 and 212 are used as lead wires when applying a cathode voltage and an image signal voltage to the pellet 206, as well as supporting the cathode structure 202 in the electron gun.
The cathode structure 202 as described above is held in a position where a center axis of the circular columnar pellet 206 is roughly in parallel with a tube axis (Z axis) of the cathode ray tube.
FIG. 1B is a cross-sectional view of the heater 208, in which the heater 208 is cross-sectioned with a plane that is perpendicular to the tube axial direction. As shown in the drawing, the heater 208 is made of a ceramic (electric insulating material) body 216 in which a heating wire 214 is partially buried. In an example given here, the heating wire 214 in the ceramic body 216 includes three coiled parts that are connected in series. Leading parts 214B are both ends of the heating wire that are extending from the ceramic body.
In the above cathode structure, the coiled parts are buried so that a lengthwise direction of each coiled part becomes perpendicular with the Z axis. Accordingly, in comparison with a common cathode structure in which the lengthwise direction of the coiled part is in parallel with the Z axis, it is possible to reduce the size of the cathode structure in the tube axial direction. In addition, in a case of the common cathode structure, heating efficiency of the electron-emitting material varies in the lengthwise direction of the coil, because distances to the electron-emitting material from one end of the coil and that from the other end of the coil are different. However, in a case of the cathode structure 202 illustrated in FIGS. 1A and 1B, it is possible to heat the electron-emitting material evenly in the lengthwise direction.
However, inventors of the present invention found that, when the cathode structure 202 illustrated in FIGS. 1A and 1B is used for an extended period of time, it often happens that an amount of electron beam cannot be controlled by the cathode voltage or the image signal voltage.
The inventors of the present invention also found out that barium (Ba) evaporated from the pellet 206 when heated causes the above problem. Specifically, the evaporated barium accumulates on a side surface of the circular columnar ceramic body 216 and such, and eventually causes a short-circuit between the leading parts 214B of the heating wire 214 and the metal cup 204 of the pellet 206. As a result, a relative potential difference between a G1 electrode (control electrode) and a cathode (pellet) corresponding to the cathode voltage and the image signal voltage cannot be obtained, and the amount of electron beam cannot be controlled.
In addition, a temperature at a part, which is just exposed from the ceramic body 216, of the leading parts 214B of the heating wire 214 becomes as high as the coiled part buried in the ceramic body 216 when operating. Accordingly, unnecessary thermal electrons are emitted as a result of an influence of the accumulated barium, independently of the cathode voltage and the image signal voltage.